The present invention relates to determining the position of data storage media relative to an access head through the use of one or more servo tracks.
Data storage systems including storage media moving relative to an access head are commonly used to store voice and data information due to the reliability, cost efficiency, and ease of use of such storage media. Media types may store information through a variety of means and may be formed in a variety of formats including magnetic tape, magnetic disks, optical tape, optical disks, and the like. Storage media may be made more useful and cost-effective by increasing the areal density of stored information. This has been accomplished by including more data tracks across a given width of the media. While allowing more data to be stored, the increase in the density of data tracks requires a narrowing of the width of the data tracks, a narrowing of the spacing between data tracks, or both. As the data tracks are more closely spaced, positioning of the media with respect to a media access head becomes more critical to reduce the possibility of errors introduced while reading or writing information.
For example, magnetic tape access head assemblies generally include read heads for reading data from the magnetic tape and write heads for writing data to the magnetic tape. Typically, read heads may be formed in a read module with one read head for each data track that is to be simultaneously read. Similarly, write heads are manufactured into a write module, with one write head for each data track to be simultaneously written. Thin film construction techniques are used to achieve the small geometries required to produce read heads and write heads capable of accessing densely packed data tracks. To permit read-after-write operation on tape moving in either tape direction over the tape access head assembly, a typical tape access head assembly may include a sandwich of one write module between two read modules.
In order to accurately position the tape access head assembly relative to the tape, one or more servo stripes or tracks may be used to provide positional reference along the length of the tape. Servo read heads in the tape access head assembly sense tracking patterns on the servo track and produce signals which are received by a control system. The control system moves the tape access head assembly to keep the servo signals at nominal magnitude. The nominal signal occurs when the servo read head is located in a certain position relative to the servo track.
The position of the servo read head relative to the servo track is typically determined from the strength of the servo read signal when the servo read head is accessing a particular portion of the servo track tracking patterns. Generally, the greater the overlap of the servo read head and a field in the tracking pattern being read, the greater the magnitude of the servo read signal. Present systems assume a linear relationship between changes in the strength of the servo read signal and corresponding changes in the percentage overlap between the servo read head and the read field. However, this assumption typically does not hold for all detectable output from the servo read head. One source of nonlinearity may by caused by read signal sensitivity effects near the edges of the servo read head. Another source of nonlinearity results from an inability to write field patterns that generate an abrupt on-off edge. Hence, what is needed is to more accurately interpret servo read head signals to determine the position error of a servo read head relative to a servo track. This results in a more accurate placement of an access head assembly relative to the media being accessed.
The present invention compensates for nonlinearities in the interaction between a servo read head and a servo track to more accurately determine the offset between the servo read head and the servo track.
A method is provided for positioning a module having at least one servo read head across media having at least one servo track. A track profile of the servo head is determined. A position error expression is determined based on the track profile. A servo track signal is generated by reading a servo track with the servo read head. A tracking error is determined based on the servo track signal and the position error expression. The module is positioned to correct the tracking error.
In an embodiment of the present invention, the track profile is determined from measurements of the servo read head taken after the module is constructed.
In another embodiment of the present invention, the position error expression is determined by assuming the servo read head track profile is trapezoidal.
In still another embodiment of the present invention, the servo track includes a checkerboard pattern with a background written at a first frequency and erase fields written at a second frequency.
In yet another embodiment of the present invention, the position error expression, PE(z), is expressed as       PE    ⁢          (      z      )        =      w    ⁢                            S          ⁢                      (                          z              -                              z                0                            -              w                        )                          -                  S          ⁢                      (                          z              -                              z                0                            +              w                        )                                                S          ⁢                      (                          z              -                              z                0                            -              w                        )                          +                  S          ⁢                      (                          z              -                              z                0                            +              w                        )                              
where S(z) is a crosstrack profile based on the servo read head track profile, w is a constant based on the width of the servo track pattern, and z0 is a measure of servo error. The measure of servo error may include errors in the servo track pattern, errors in head alignment, and the like. The crosstrack profile may be found by convolving the servo read head track profile with a servo track write profile.
A method of determining the position of an access module relative to a servo track is also provided. The servo track is read by a servo read head on the module. The servo read head generates a read signal based on a portion of a track pattern covered by the servo read head. The module position is determined based on a model of a crosstrack profile formed by varying overlap of a track profile for the servo read head and a track profile for the track pattern.
In an embodiment of the present invention, the model of the crosstrack profile is a phenomenological model. The phenomenological model of the crosstrack profile may be the convolution of the servo read head track profile and the track pattern track profile.
A system for positioning an access module relative to data storage media having at least one servo track is also provided. A servo read head, fixed to the access module, generates a read signal in response to patterns read from a servo track. A module positioning servo positions the module relative to the media based on a module control signal. Memory holds coefficients of a crosstrack profile based on a track profile for the servo read head. Control logic receives the read signal, calculates a position error based on the received read signal and the coefficients, and generates the module control signal to reduce the position error.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connect with the accompanying drawings.